Introduction:
Where windows meet exterior walls, particularly on dormer builds, the risk rises. Water can track behind the window frame, slip behind the cladding, and end up soaking into the timber structure below. It’s a detail that’s easy to overlook. Mistakes here go unnoticed early yet lead to serious damage later.
A window tray flashing is the component designed to stop that happening. If you’re building or specifying a dormer or clad wall, understanding how this piece works, and which type you need, can save time, money, and future repairs.
A window tray flashing is a pre-formed lead tray that sits beneath a window where it meets a clad external wall, most commonly on a dormer, but also on any timber-framed structure clad with slates, tiles, shiplap or similar. Its job is to collect any water that gets behind the window frame and direct it away from the structure, turning it over the face of the cladding below rather than letting it soak in.
Typical design
A standard window tray flashing includes:
• A flat base that sits beneath the window
• Upstands at the back and sides to contain water
• A front lip that laps down over the cladding
Material specification
• Code 4 lead is the industry standard for this application, thick enough for durability, flexible enough for forming
• Manufactured to BS EN 12588 for consistency and quality
Overlap requirements
• Where sections join, a minimum 100mm overlap is required
• This ensures continuity and maintains watertight performance
Trays are frequently necessary when dealing with dormer windows, specifically dormers clad in slates, tiles, or boards rather than rendered or tiled all the way around.
However they’re not limited to just dormers, any window set into a clad stud wall construction at roof level can need one.
What about masonry construction?
In traditional brick or block construction, this aspect is addressed through specific methods:
• Cavity trays
• Damp proof courses (DPCs)
Yet when dealing with timber frames alongside cladding, a window tray remains the usual choice, trusted, steady in performance. Its track record stands firm.
Leadworx supply three types of window tray:
Full length window tray
• A single piece spanning the full width of the window
• No joints, fewer potential failure points
• Simplest to install
Typical spec:
• 150mm cill depth
• 150mm front cover lap
Best for:
• Standard-width openings
• Jobs where handling a single piece is practical
Window tray halves
• Supplied as two sections
• Joined at the centre with a minimum 100mm overlap
Why use them?
• Easier to handle on wider openings where the opening is wider than a practical single piece
• Useful where site access is restricted
Same spec as full length, just split for practicality
Window tray ends (pair)
• Supplied as left and right end sections only
• The central flat section is formed on site from lead sheet by a roofer
Best for:
• Traditional roofing workflows
• Roofers comfortable dressing lead
• Projects where flexibility is needed on site
The ends handle the most complex part (corners and upstands), while the centre is cut and fitted to suit.
All three options are:
• Made from Code 4 lead
• Compliant with BS EN 12588
• Manufactured in Cornwall
• Available in standard and bespoke sizes. Leadworx often makes bespoke trays with a welded rear upstand.
Getting the installation right is what makes a window tray flashing actually do its job. These are the core details that matter on site:
• Maintain minimum 100mm overlap where sections join. This is essential for maintaining a continuous watertight barrier.
• Tray must turn up behind the window frame (not just sit under it) to form a watertight upstand. This is essential to stop water running back into the building.
• Front ledge of the tray should turn down over the face of the cladding, and extend to cover fixings and the cladding edge
• Avoid fixing the window frame directly through the lead tray.
• The Lead Sheet Association guidance should be followed for full specification detail
• Bespoke sizes are available if standard sizes don’t fit the opening
• Undersizing the tray: You need enough depth (typically 150mm) to form a proper upstand. Undersized trays are one of the most common causes of failure. Measure twice!
• Insufficient overlap: 100mm is the minimum; more is safer
• Poor front detailing: The front edge must extend far enough to cover cladding fixings
• Fabricating from flat sheet when pre-formed trays are available: Pre-formed trays save time and reduce the risk of weak corners
Conclusion
Finding the correct window tray flashing comes down to a practical decision:
• Full length for simplicity and speed
• Halves for wider openings or easier handling
• Ends for traditional, on-site formed solutions
All achieve the same goal of blocking water, but the correct type makes installation smoother and more reliable.
A bespoke tray might work better if your space is unusual. When measurements fall outside of typical ranges, custom options may be necessary. Bespoke trays can be made to order, ensuring a proper fit and performance.
Next step: Explore the Window Trays range to find the right configuration for your project
